178 PHYTOTOMY. 



fibres, of extreme fineness. But we must distinguish this 

 form into the primitive and the derived. 



278. 



The primitive spiral-form consists of canals, the diameter 

 of which is almost of the same size throughout, and is from 

 the twelfth to the fiftieth part of a Hne, their sides being 

 composed of those winding fibres, which can easily be un- 

 rolled ; (Tab. IV. Fig. 19.; Tab. V. Fig. 1.) We find an 

 instance of this form in some Confervae, in some of the Musci 

 Hepatici, (Tab. III. Fig. 8.) ; and especially in the cellular 

 texture which covers the surface of the Sphagnum obtusifo- 

 Hum; (Tab. III. Fig. 25.) 



The fibres which, by their windings, form the sides of 

 the spiral canals, have so uncommonly small a diameter, that 

 we might suspect them to be any thing but hollow. Several 

 of them, however, especially in the Scitamineae, commonly 

 stick together, and in this way they are formed into bands ; 

 (Tab. IV. Fig. 19.) They are also easily unrolled, so long 

 as they are in their primitive state, because there is no con- 

 necting membrane, either external or internal, by which 

 they are united ; and this is the chief distinction between the 

 air-vessels of insects and the spiral-vessels of plants, that the 

 former have the winding fibres united by a peculiar mem- 

 brane, and that a soft cellular texture always surrounds 

 them. 



279. 

 But a still more important circumstance essentially distin- 

 guishes the spiral-vessels of plants from the air-vessels of in- 

 sects. The former never divide into branches ; but where 

 they separate, a new pair always places itself on the sides of 

 the old ones, whilst the air-vessels of insects undergo every 

 kind of ramification, from their origin to their finest branches. 

 The primitive spiral-vessels are always in the company of the 

 sap-vessels, and are chiefly found between the bai'k and pith, 

 in the common plants, which are produced with two seed 

 lobes. But they appear later than the sap-vessels, and are 



